The invention relates to a method for controlling a receiver that is implemented on the RAKE principle and comprises a number of correlators which are able to synchronize with a received signal.
A receiver operating on the RAKE principle comprises several branches, and each branch can synchronize with a different signal component. The receiver can thus receive a plurality of signals simultaneously. RAKE receivers are used especially in CDMA receivers.
CDMA is a multiple access system based on a spread spectrum technique, and it has recently been put into use in cellular radio systems in addition to previously used FDMA and TDMA. CDMA has many advantages over the prior methods, such as simplicity of frequency planning, and spectrum efficiency.
In a CDMA method, a narrow-band data signal of a user is multiplied to a relatively broad band by a spreading code having a much broader band than the data signal. Band widths used in known test systems include e.g. 1.25 MHz, 10 MHz and 25 MHz. The multiplication spreads the data signal over the entire band to be used. All the users transmit simultaneously on the same frequency band. On each connection between a base station and a mobile station is used a different spreading code, and the signals of the users can be distinguished from one another in the receivers on the basis of the spreading code of the user. If possible, the spreading codes are selected in such a way that they are mutually orthogonal, i.e. they do not correlate with one another.
Correlators in conventionally implemented CDMA receivers are synchronized with a desired signal, which they recognize on the basis of the spreading code. In the receiver the data signal is restored to the original band by multiplying it by the same spreading code as in the transmission step. Ideally, the signals that have been multiplied by some other spreading code do not correlate and are not restored to the narrow band. In view of the desired signal, they thus appear as noise. The object is to detect the signal of the desired user from among a number of interfering signals. In practice, the spreading codes correlate, and the signals of the other users make it more difficult to detect the desired signal by distorting the received signal non-linearly. This interference caused by the other users is called multiple access interference.
It is vital to the performance of the spread spectrum system that the receiver is able to synchronize with an incoming signal quickly and accurately. The synchronization with the incoming signal usually takes place in two steps. In code phase acquisition, the aim is to find the desired signal in the input and determine its phase with the accuracy of half a chip. When this has been accomplished, the phase is considered locked, and the code phase is then fine-adjusted with a code tracking loop, which maintains the phase lock.
The code phase acquisition can be implemented on either the applied filter or active correlation principle. The former method is rapid, but it can be utilized only with short codes and, when implemented digitally, it requires much current. Active correlation is the most generally used method in CDMA systems. In active correlation, the code phases of a local correlator are monitored at half-a-chip intervals and compared with the received signal. This is economical, but slow. The acquisition can be speeded by using several correlators in parallel, whereby the acquisition area can be divided into sections. The acquisition time is then naturally shortened.
In earlier solutions, RAKE receivers are designed such that the RAKE branches have fixed modes of operation. An acquisition branch looks for signals addressed to the receiver, and separate correlators are reserved for tracking and demodulation of the found signals. Another known solution is that all correlators are used for acquisition in establishing a connection with the system, but when the desired signal has been found, the correlators have fixed modes of operation, i.e. one or two branches are used in the acquisition and the others in the tracking of the desired signal.
In known solutions, the operation of the RAKE branches is not adaptive but either fixed or predetermined. The present receivers are designed to operate primarily in macro cells, i.e. large cells, in which the propagation delays of multipath-propagated signals are long and several tracking correlators are needed. In macro cells, there is time to look for a new base station signal, since the cells are large and a need for changeover does not arise unexpectedly. Micro cells, on the other hand, are small, often less than 500 m. A transmitter and a receiver often have a line of sight, and so the main part of the energy of the signal is contained in the direct propagated component. There is often no need to track several multipath-propagated components. In the solutions of the prior art, the tracking correlators of the receivers are thus not in use.